fix: harden scans, rules, Civitai parsing, open_directory, aria2 lifetime

[mogurt]

Summary

Batch 3 of the review items: backend robustness.

• Directory scans
  • scan_workflow_dir and check_models_local now run on a blocking thread instead of the async runtime.
  • Both use one walker that still follows symlinks and junctions, which are common for ComfyUI model folders. It visits each canonical directory once and caps the depth, so link cycles no longer recurse until the stack overflows.
• open_directory only accepts directories. A file path would have been opened or executed by explorer / open.
• Custom rules (suggest_type)
  • Malformed rules JSON is now an error.
  • A single rule that doesn't parse, or that names an unknown model type, is skipped with a warning. Before, unwrap_or_default silently discarded every rule.
• Civitai
  • The model-version metadata status is checked before parsing; a 401/404 JSON body used to be treated as metadata.
  • Content-Disposition parsing prefers filename*, is case-insensitive, and no longer percent-decodes plain quoted names.
• aria2 lifetime: aria2 is started with --stop-with-process=<app pid>, so it exits by itself when the app crashes or is killed. kill_on_drop never runs in that case, and the orphaned process kept file locks.

Testing

• cargo fmt --check
• cargo test: 23 passed on Windows, plus a Unix-only symlink-cycle test that CI's macOS job runs. New tests cover the directory walker, rule parsing and Content-Disposition variants.

End-to-end on Windows (tauri dev with a separate identifier and data dir):

• Junction cycles (mklink /J back to an ancestor, in both the workflows and the models tree):
  • scan_workflow_dir returns wf.json in 4 ms.
  • check_models_local finds the nested model and reports the missing one, in 3 ms.
• open_directory with a file path is rejected with "Not a directory".
• suggest_type
  • A higher-priority rule with type bogus is skipped (warning logged) and the valid rule still applies (lora).
  • {broken JSON is rejected with an explicit error.
• Civitai URL parse (/api/download/models/9208) still returns the filename, type, size and hash.
• aria2 lifetime: after force-killing the app process, aria2c (started with --stop-with-process=<pid>) exited on its own within ~2 s.